River Flood Warning 4, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
745 PM PST Sun Jan 8 2023

.SYNOPSIS...Rain and strong winds will spread across the region
tonight, lasting through tomorrow morning. The threat of showers and
thunderstorms will continue through the day, with more widespread
rain and winds moving into the region Monday night through Tuesday.
Expect additional impacts including widespread flooding, damaging
winds, and dangerous marine conditions. Please stay current with
forecast updates as there is a direct threat to life and property
from these storms.

&&

...Increased Wind and Flooding Risk into Early Next Week...

.DISCUSSION...As of 04:05 PM PST Sunday...Water vapor imagery
continues to show Northern California between systems this
afternoon, with a plume of moisture and clouds pointed right at
the state. Given the partly sunny skies this afternoon,
temperatures are running up to 9 degrees warmer this afternoon
than this time yesterday. All in all, it is a great day and
beautiful day to prepare for the next big Pacific storm headed our
way tonight.

This powerful Pacific storm will bring multiple impacts to the
region, but come in waves of rain and wind between tonight and
Tuesday. The first and stronger wave will impact the region tonight
through early Tuesday.

As for the rain, light rain will initially spread across the
region this evening, but intensify as we go through the late night
and early morning hours. TPW imagery shows that the moisture plume
feeding into this first band upwards of 1.2 to 1.4 inches, so
there is plenty of moisture available. Given the moisture content
and the instability for tonight, periods of heavy rain will be
possible late tonight through tomorrow morning. The models have
been speeding up this portion of the system, moving it through
the majority of the Bay Area by 10 am. The exception is Monterey
County, where we will likely be in it a little longer. That said,
the morning commute will be a mess with the high winds, heavy
rain, and likely flooding. Travel is not advised tomorrow morning.
During this time frame, the WPC Excessive Rainfall Outlook paints
the entire forecast area with a moderate risk, which suggests a
40-70% chance of exceeding Flash Flood guidance.

Even though the models are also suggesting a break in the
rainfall tomorrow afternoon, believe showers will continue
through the day. SPC also has the Central Coast in Marginal Risk
for severe weather, as there is instability with strong low level
wind could lead to a brief tornado. The secondary wave will bring
another round of heavy rain to the region Monday night through
Tuesday morning. This secondary system is even more unstable than
the first, with showers and thunderstorms likely continuing after
the mainband of rain. These showers and thunderstorms will be
capable of putting down significant rainfall in a short amount of
time.

Therefore, a Flood Watch remains in effect through 4 pm on
Tuesday. Do expect widespread flooding to occur late tonight and
through Tuesday, with rapid rises along area streams and creeks
and rivers will occur. Several mainstem rivers are also forecast
to go above flood stage as well, and Flood Warnings have started
to be issued. A Flash Flood Watch has also been issued for the
Monterey County burn scars of the River, Colorado, and Dolan
Fires. In coordination with our partners at the USGS, the threat
for widespread shallow landslides is likely, especially along
steep hillslopes during intense precipitation.

As for the winds tonight through Monday morning: Strong winds
will begin to spread across the region between 8 and 10 pm,
strengthening and continuing through the night and early morning
hours, before weakening. Our local wrf as well as the Nam12 show
925 winds increasing to between 60 and 70 kts, with even strong
winds at 850 mbs. Therefore, a High Wind Warning is now in effect
starting at 8 pm this evening. Expect gusts up to 60 mph possible
along the immediate coast and our valley locations. Gusts up to 80
mph will be possible in the higher elevations, and it would not
surprise me if we had some local gusts up to 90 mph in some of the
favored windy spots. Winds will decrease during the morning
hours. That said, winds will increase again Monday night, and more
wind products will likely be needed at the time.

Drier conditions will finally develop for Wednesday and Thursday for
most of the area. However, models are suggesting that a warm
front may clip the region. That said, think the North Bay would be
the region most likely to see some light rain those days. Any
rain would be light, with most areas seeing less than a quarter
inch. However, North Bay could see between 1 to 2 additional
inches with the coastal mountains seeing up to 3. Another weak
system will then move into the region on Friday. Followed closed
by another more significant system for the weekend. At this time,
there are still some model differences, but given the fact that
most areas will not have had sufficient time to dry out, flood
watches will likely be needed again. Palmer

Some of the more notable river and stream forecasts that are
expected to reach at least minor flood stage include, but not
limited to, the Russian River at Guerneville, Alameda Creek near
Niles, Coyote Creek above HWY 237 at Milpitas, and the Big Sur
River near Big Sur. More notable is the San Lorenzo River at Big
Trees, which is forecast to reach major flood stage Monday
evening.

**FORECAST RAINFALL TOTALS TONIGHT - TUESDAY**

North Bay
- lower elevations 3 to 4 inches
- higher elevations 4 to 6 inches

San Francisco & SF Peninsula
- lower elevations 2 to 4 inches
- higher elevations 4 to 6 inches

East Bay
- lower elevations 2 to 3 inches
- higher elevations 3 to 5 inches

South Bay
- lower elevations 2 to 3 inches
- higher elevations 3 to 6 inches (highest in the western terrain of
Santa Clara County)

Santa Cruz County
- lower elevations 4 to 6 inches
- higher elevations 6 to 9 inches

Monterey County & San Benito County
- lower elevations 2 to 4 inches
- Big Sur Mountains 5 to 10 inches

&&

.AVIATION...As of 7:45 PM Sunday...For the 00Z TAFs. Rapidly
deteriorating weather conditions tonight with a fast (~ 50 mph)
moving cool front arriving from the west later tonight and Monday
morning. Expect very strong winds and low level wind shear. 12z
statistical guidance for 09z-12z Monday morning forecasting 40+
knots southeast surface winds at SFO and OAK, the newly arrived
00z guidance is lower at ~ 30 knots, nonetheless other forecast
guidance shows a fast moving frontal system. Strong winds veering
(turning clockwise) with altitude up until cool frontal passage,
any instability that strengthens may result in rotation in
thunderstorm(s). Will monitor radar, satellite, observations for
thunderstorm develoment overnight and amend TAFs as needed. Otherwise
rain, heavy at times up through cool frontal passage, the front
lastly to move through along the Big Sur Coast Monday. Next
upstream trough over the Pacific arrives later Monday evening

Vicinity of SFO...VFR-MVFR, evening southeast wind 10 knots,
however winds quickly increasing mid to late evening with gusts
reaching 50 knots or potentially higher 09z-13z Monday. Very
strong low level wind shear tonight and Monday morning. Airport
weather warning for winds has been issued. Rain, heavy at times
tonight and Monday morning up through cool frontal passage. South
to southwest winds 15 to 20 knots Monday morning. Showers continuing
through most of the day with VFR and lighter winds. Showers arriving
again after 06z (10 pm PST Monday) Tuesday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay...VFR-MVFR, evening southeast winds 5 to 15 knots,
however winds quickly increasing mid to late evening with gusts
reaching 30 to 50 knots overnight and Monday morning up until
cool frontal passage. Very strong low level wind shear tonight and
Monday morning. Rain tonight and Monday morning up through cool
frontal passage. Southwest winds 10 to 15 knots during the day
Monday. Showers returning late Monday night and Tuesday morning.

&&

.MARINE...As of 2:07 PM PST Sunday...Another round of rain and
strong winds with gale force gusts arrives this evening into
Monday lasting through Tuesday. Active weather continuing beyond
midweek next week. Longer period westerly swell arrives at the end
of the work week.

&&

.HYDROLOGY...As of 4:05 PM PST Sunday...Expect significant
hydrologic impacts over the next 24 to 48 hours.

Current Flood Warnings in effect...
Guadalupe River above the Almaden Expressway in San Jose
The Carmel River at Robles Del Rio

RVS is effect:
Napa at Napa
Napa at Saint Helena

Flash Flood Watch is now in effect for the Monterey County burn
scars, specifically the Dolan, Colorado, and River burn scars.

Flood Watch for mainstem river flooding remains in effect for
the...
Russian River at Guerneville
Pajaro River at Chittenden
Salinas River at Spreckles

Areal Flood Watch remains in effect for all other area rivers and
streams and creeks, including but not limited too.
In North Bay...
Russian River at Geyserville
Mark West Creek near Mirabel Heights
Maacama Creek near Kellogg
Laguna de Santa Rosa at Stony Point Road near Cotati

In East Bay...
Alameda Creek near Niles
Alameda Creek above Div Dam near Sunol
Arroyo de la Laguna at Verona

In South Bay...
Guadalupe River above Hwy 101 at San Jose
Coyote Creek above Hwy 237 at Milpitas
Coyote Creek at Edenvale
Pacheco Creek near Dunneville

In the Santa Cruz Mountains...
San Lorenzo River at Big Trees

In Monterey County...
Salinas River near Bradley
Big Sur River at Big Sur
Arroyo Seco near Soledad
San Lorenzo Creek below Bitterwater Creek near King City
San Antonio River near Lockwood

In San Benito County...
San Benito River at Highway 156 near Hollister
Tres Pinos Creek near Tres Pinos

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Flood Watch...CAZ006-502>506-508>510-512>518-528>530
             High Wind Warning...CAZ006-508>510-512>515-529
             High Wind Warning...CAZ502>506
             High Wind Warning...CAZ516>518-528-530
             Flash Flood Watch...CAZ517-528-530
             GLW...SF Bay from 9 PM
             GLW...Mry Bay from 9 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 9 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 9 PM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm from 9 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 9 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 9 PM

&&

PUBLIC FORECAST: Palmer
AVIATION: Canepa
MARINE: Murdock

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